Speed limits put in place by Network Rail as a safety measure have left a number of platforms out of action.
Operator London Northwestern Railway promised updates ahead of the evening rush hour.
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A spokesman said: “We've been told that due to a speed restriction because of high track temperatures between Rugby and Milton Keynes Central, we are unable to use several platforms at Northampton so services will have to terminate short.
“Disruption is expected until the end of the day.
“Road transport has been requested and will shuttle between Rugby and Northampton and between Milton Keynes and Northampton.”
Travellers on road and rail have already been warned to prepare for days of potential disruption as temperatures rise above into high 30s this weekend.
A Network Rail spokesman said: “Tracks can buckle in hot weather making it unsafe for trains to pass over.
"By slowing trains down, they exert lower forces on the track reducing the chance of this happening.
“Heat can also cause overhead wires to sag so trains have to be slowed down to ensure they do not risk damaging them.”
Forecasters predict slightly cooler weather for Northamptonshire over the next couple of days, although still in the high 20s, followed by extreme heat with the possibility of thermometers reaching 40°C in some places.
Met Office experts have extended a rare amber extreme heat alert from Sunday (July 17) into Monday.